Rolling resistance of tires is mainly caused by energy losses due to repeated deformation of the rubbers during running. In order to reduce the rolling resistance, for example, a structure has been proposed in which a tread, which contributes the most to the rolling resistance, has two rubber layers, with the inner one (base tread) formed from a rubber composition causing a small energy loss and the outer one (cap tread) formed from a rubber composition having excellent abrasion resistance. Thus, rubber compositions used for the base tread are required to have superior fuel economy to rubber compositions used for the cap tread. Additionally, the rubber compositions for the base tread are required to have excellent handling stability, elongation at break, and durability.
In order to satisfy these requirements, for example, a rubber composition for a base tread is used which includes a rubber component composed of an isoprene-based rubber, butadiene rubber and a modified styrene butadiene rubber, and also includes zinc oxide typically in an amount of 2.5 to 5 parts by mass relative to 100 parts by mass of the rubber component.
Meanwhile, these days, zinc oxide included in tire rubber compositions has been regarded as a problem from the viewpoint of environmental pollution, and thus reduction of the zinc oxide content is desired. However, the reduction of the zinc oxide content tends to deteriorate fuel economy, handling stability, elongation at break, and durability. For this reason, the reduction of the zinc oxide content has been practically difficult.
Various attempts have been made to enhance the performances required for the base tread. For example, the fuel economy can be significantly enhanced by reducing the filler content and adding a modified polymer such as modified styrene butadiene rubber. However, this arrangement causes a significant decrease in elongation at break (especially, elongation at break at high temperatures), and therefore adding a large amount of zinc acid is necessary. Thus, further improvement is still needed to enhance fuel economy and elongation at break while reducing the amount of zinc oxide.
Patent Document 1 discloses the use of Tackirol V200(product of Taoka Chemical Co., Ltd.) as a hybrid crosslinking coagent. This arrangement can enhance fuel economy; however, disadvantageously, the dispersibility of Tackirol V200 is low. Therefore, further improvement is required for enhancing fuel economy, elongation at break and durability in a balanced manner while maintaining favorable handling stability and processability (extrusion processability). Moreover, further improvement is required for reducing the amount of zinc oxide.    Patent Document 1: JP 2009-84534 A